Spectrum sensing engine

ABSTRACT

Systems, methods, and devices for reducing interference with digital television transmissions occurring over a bandwidth are disclosed. The digital television signal is correlated to a reference digital television field sync signal. A non-coherent correlation power measurement is determined based on the correlation of the received digital television signal to the reference digital television field sync signal. A plurality of maximum non-coherent correlation power measurements are determined over multiple field times. An energy estimate for the digital television transmission is determined based on the maximum non-coherent correlation power measurements. A transmit mask filter is generated based on the energy estimate. The transmit mask is applied to transmissions to reduce interference with detected digital television transmissions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of, and claims priorityto, U.S. patent application Ser. No. 12/620,690, entitled SPECTRUMSENSING ENGINE, to inventors Gossett et al., which was filed on Nov. 18,2009 and claims the benefit under 35 U.S.C. §119(e) of U.S. PatentApplication Ser. No. 61/163,380 entitled SPECTRUM SENSING ENGINE, toinventors Gossett et al. which was filed on Mar. 25, 2009, thedisclosures of the foregoing applications are incorporated herein byreference in their entirety.

BACKGROUND

This document relates to electromagnetic spectrum allocation andutilization.

The wireless spectrum is usually available to any wireless device.However, while the wireless spectrum can generally be used by anywireless device, the devices that operate in the wireless spectrumshould respect other transmissions that are occurring in the samespectrum. Examples of the other transmissions are digital televisionsignals. Wireless devices can respect other transmissions by operatingin a manner that reduces interference with the other transmissions.Interference can be reduced by detecting transmissions in portions ofthe spectrum and attenuating transmissions in those portions of thespectrum occupied by the detected transmissions.

SUMMARY

In general, one aspect of the subject matter described in this documentcan be embodied in methods that include the actions of correlating areceived digital television signal to a reference digital televisionfield sync signal; determining a non-coherent correlation powermeasurement based on the correlation of the received digital televisionsignal to the reference digital television field sync signal;accumulating a plurality of maximum non-coherent correlation powermeasurements over a plurality of field times; determining an energyestimate based on the maximum non-coherent correlation powermeasurements; and generating a transmit mask filter based on the energyestimate. Other embodiments of this aspect include correspondingsystems, apparatus, and computer program products.

These and other implementations can optionally include one or more ofthe following features. The method can include the actions of shifting acenter frequency of the received digital television signal to areference frequency; filtering the received digital television signal toselect a portion of the received digital television signal; andconverting a sample rate of the selected portion of the received digitaltelevision signal to a system sample rate.

The received digital television signal can be an Advanced TelevisionSystems Committee standard digital television signal. The filter can bea 3 MHz low-pass filter. The correlating can be implemented as theaction of filtering the received digital television signal with a finiteimpulse response filter. The correlation power can be determined throughthe action of determining a windowed maximum power of the correlation ofthe received digital television signal to the reference digitaltelevision field sync signal.

The windowed maximum power determination can be performed through theactions of determining the windowed maximum power of the correlation ofthe received digital television signal to the reference digitaltelevision field sync signal on a first window of data; and determiningthe windowed maximum power of the correlation of the received digitaltelevision signal to the reference digital television field sync signalon a second window of data, wherein the second window of data comprisesa portion of the data from the first window of data.

Particular implementations of the subject matter described in thisdocument can be implemented so as to realize one or more of thefollowing advantages. Digital television transmissions can be detectednon-coherently. Transmissions can be attenuated in a portion of thespectrum occupied by a digital television transmission. Availablebandwidth for using a wireless device can be dynamically determined.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example illustration of devices operating in a spectrum.

FIG. 2 is a block diagram illustrating an example spectrum sensingengine.

FIG. 3 is a block diagram of an example modulator.

FIG. 4 is a block diagram of an example energy extractor.

FIG. 5 is a flow chart of an example process of spectrum sensing.

FIG. 6 is a flow chart of an example process of determining a windowedmaximum power.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

§1.0 Example Wireless Environment

FIG. 1 is an example illustration of devices using a spectrum 102. Thespectrum 102 can be used, for example, by a television broadcaster 103that is transmitting a digital television signal. The digital televisionsignal can be, for example, an Advanced Television Systems Committee(“ATSC”) standard signal. Each ATSC signal is a licensed use of thespectrum 102 and has priority to transmit in corresponding definedportions 105 of the spectrum 102. However, the number of ATSC signalsand the corresponding defined portions 105 of the spectrum 102 occupiedby ATSC signals can vary across geographic regions. Similarly, thecorresponding defined portions 105 of the spectrum 102 that are occupiedby ATSC signals within a particular geographic region (e.g.,transmission footprint) can vary over time as television broadcasters103 add television channels in the particular geographic region.

While ATSC signals have priority to use the spectrum 102, wirelessdevices 104 can operate (e.g., transmit) in unused portions 106 of thespectrum 102. However, transmissions from the wireless devices 104cannot interfere with the ATSC signals that are using correspondingdefined portions 105 of the spectrum 102. For example, each of thewireless devices 104 a-104 e can simultaneously use portions 106 a-106 eof the spectrum 102, respectively, as long as the wireless devices 104a-104 e do not interfere with the ATSC signals occupying thecorresponding defined portions 105.

The wireless devices 104 a-104 e can be fixed or mobile. Mobile wirelessdevices 104 c and 104 d (e.g., mobile phones, portable computers, PDAs,etc) can travel in and out of geographic areas that have different ATSCsignals occupying the spectrum 102. Therefore, the mobile wirelessdevices 104 c and 104 d can begin transmitting in a new geographic areaat portions of the spectrum 102 that are not occupied by ATSC signals.Similarly, new wireless devices 104 can begin transmitting in thespectrum 102 in a given geographic region where broadcasters aretransmitting ATSC signals.

When each of the wireless devices 104 a-104 e are transmitting indistinct corresponding portions 106 a-106 e of the spectrum 102, therewill not be interference between the wireless devices 104 a-104 e andthe ATSC signals that are using the corresponding defined portions 105of the spectrum 102. Similarly, when a new wireless device enters thenetwork and selects a portion 106 a-106 e of the spectrum 102 notoccupied by an ATSC signal, there is no interference between the newwireless device and the ATSC transmissions that occupy the portions 105.

However, without identifying the defined portions 105 of the spectrum102 that are occupied by ATSC transmissions and avoiding these definedportions 105, the new wireless device may transmit in a defined portion105 of the spectrum 102 that is already occupied by an ATSC signal and,in turn, cause interference with the ATSC signal. Similarly, when mobilewireless devices 104 c and 104 d enter a new geographic region, themobile wireless devices 104 c and 104 d may interfere with ATSCtransmissions in portion 105 that were not present in portion 105 in theprevious geographic region. Providing data to the wireless devices 104a-104 e that identifies defined portions 105 of the spectrum 102 thatare occupied by ATSC transmissions can prevent interfering transmissionsfrom the wireless devices 104 a-104 e.

In some implementations, a spectrum sensing engine 110 can be coupledwith the wireless devices 104 a-104 e that provide data identifying theoccupied defined portions 105 of the spectrum 102. In turn, the spectrumsensing engine 110 can generate a transmit mask for each wireless device104 a-104 e that precludes each wireless device from transmitting in anoccupied defined portion 105 of the spectrum 102 that would causeinterference with ATSC transmissions.

In some implementations, the spectrum sensing engine 110 can beimplemented in stationary wireless devices (e.g., desktop computers,routers, repeaters, etc.). In some implementations, the spectrum sensingengine 110 can be implemented in mobile wireless devices (e.g. laptopcomputers, mobile phones, wireless microphones, etc.). The spectrumsensing engine 110 can be implemented, for example, as a component thatis embedded in the wireless devices or as an external module thatconnects to the wireless device through a communications interface(e.g., USB, Ethernet, RF interface, optical interface, or any othercommunications interface).

In some implementations, the spectrum sensing engine 110 can identifyATSC transmissions in the spectrum 102 by extracting the energy from anATSC channel (e.g., defined portions 105 and/or 106 a-106 e of thespectrum that corresponds to an ATSC channel band) and determine anon-coherent (e.g., non-synchronized) correlation between the extractedenergy and known ATSC field synchronization signals. By extracting thetotal energy from an ATSC channel, the spectrum sensing engine canidentify the presence of an ATSC transmission without recovering a fieldsync signal that is associated with the ATSC transmission. Therefore,ATSC transmissions can be detected at a lower signal-to-noise ratio(“SNR”) than otherwise possible.

§2.0 Example Spectrum Sensing Engine

FIG. 2 is a block diagram illustrating an example spectrum sensingengine 110. The spectrum sensing engine 110 can include a modulator 202,a sample rate converter 204, a field sync correlator 206, and an energyextractor 208.

In operation, the spectrum sensing engine 110 can receive a signal atthe modulator 202. The received signal can be, for example, a digitizedversion of an RF transmission. The modulator 202 can select the portionof the received signal (e.g., the ATSC channel) to be analyzed by tuningto the center frequency of the ATSC channel and filtering sidebandsignals to isolate the ATSC channel. The output of the modulator 202 isthe portion of the received signal to be analyzed in quadrature. Eachquadrature output of the modulator 202 can be passed to a sample rateconverter 204 (SRC).

The sample rate converter 204 can be used to convert the sample rate ofeach quadrature output of the modulator 202. The real and imaginarycomponents of the quadrature output from the modulator 202 can each bereceived by a corresponding sample rate converter 204. In turn, eachsample rate converter 204 can convert the sample rate of each quadratureoutput.

In some implementations, the sample rate converter 204 can convert thesymbol rate at the output of the modulator 202 from a system sample rate(e.g., 122.2 MHz) to twice the symbol rate of an ATSC signal (e.g.,2*10.7622 MHz) so that an ATSC signal can be accurately decoded. In someimplementations, an integer relationship does not exist between theinput and the output of the sample rate converter 204. In theseimplementations, the output is valid after a variable number of systemclock cycles that can be determined based on the input symbol rate andthe output sample rate.

Each sample rate converter 204 can be implemented with a low orderfilter (e.g., fourth order) because the input to the sample rateconverters 204 has already been low-pass filtered by the modulator 202and a relatively large conversion is being performed (e.g., 122.MHz/20.12 MHz). Additionally, 12 bits of precision are required suchthat look up table interpolation may not be necessary. When look uptable interpolation is used, the sample rate converter 204 can beimplemented to accumulate 32 bits of the phase. The 12 most significantbits of the phase can be used as the phase input to a filter coefficientlookup table.

The output of each sample rate converter 204 can be passed to the fieldsync correlators 206. The field sync correlator 206 can receive theoutput of the sample rate converter 204. In some implementations, realand imaginary outputs can be received from sample rate converters 204.In these implementations, the real and imaginary outputs from the samplerate converters 204 can each be received by a corresponding field synccorrelator 206. Each field sync correlator 206 filters the receivedsignals using a Finite Impulse Response (“FIR”). In turn, the output ofeach field sync correlator 206 increases when an ATSC signal isreceived.

In some implementations, the field sync correlators 206 can determine anon-coherent correlation between the received signal and a referencepseudo-random ATSC field sync signal (e.g., reference digital televisionsignal) that is modeled, for example, by taps of a FIR filter. Forexample, each field sync correlator 206 can be implemented, as a FIRfilter. The taps of the FIR filters can correspond to the pseudo-randomATSC field sync signal to model the reference ATSC field sync signal.Accordingly, when an ATSC field sync signal is received at the input ofthe FIR filters, there will be an increase in the magnitude of theoutput of the FIR filters.

The output of each field sync correlator 206 can be passed to the energyextractor 208. The energy extractor 208 can receive the quadratureoutputs from the field sync correlators 206 and accumulate a maximumcorrelation power over ATSC field times. In some implementations, theenergy extractor 208 can identify a maximum correlation power bymeasuring the maximum power in predefined windows (e.g., time windows)of the ATSC field time and accumulating these maximum power measurementsinto bins that correspond to each window of ATSC field time. When anATSC signal is present, one or more bins will have a power magnitudethat defines a power spike relative to the other bins. This power spikeindicates the presence of an ATSC field sync signal and, in turn, anoccupied portion 105 of the spectrum 102. When the power spike isdetected, a transmit mask filter can be implemented to preventtransmissions in the occupied portion 105 of the spectrum 102 that wouldinterfere with the received signal.

FIG. 3 is a block diagram of an example modulator 202. In someimplementations, the modulator can be implemented as a Weaver modulator.A Weaver modulator can tune an ATSC channel by shifting the centerfrequency of the ATSC channel to DC. The Weaver modulator can shift thecenter frequency, for example, by mixing the received signal with a lowfrequency oscillator 302. In some implementations, the shift can beperformed to produce a quadrature pair by splitting the received signaland applying a cosine modulation to one tap of the received signal,while applying a sine modulation to the other tap of the receivedsignal.

Each of the quadrature signals can be passed to a low pass filter 304.The low pass filters 304 can be implemented with the same filtercharacteristics so that the quadrature pair is filtered in the samemanner. In some implementations, the low pass filters 304 areimplemented as 3 MHz low pass filters (e.g., 6 MHz ATSC channel width/2)because the ATSC channel is centered at DC. This low pass filteringremoves unwanted side-bands from the received signal.

The outputs of the low pas filters 304 can be mixed with a highfrequency oscillator 306. In some implementations, the high frequencyoscillator 306 can include a quadrature pair of oscillators. Each of thelow pass filter outputs can be mixed with the sine and cosineoscillators to produce a real and imaginary component for eachquadrature pair. In turn, the real component from each quadrature paircan be summed and the imaginary component from each quadrature pair canbe subtracted to obtain a quadrature output. As discussed above, theoutput from the modulator 202 can be passed to the sample rate converter204.

Although the example modulator 202 shown in a Weaver modulator, othermodulators that output the signals described above can also be used.

FIG. 4 is a block diagram of an example energy extractor 208. In someimplementations, the energy extractor 208 can extract a maximum powerfrom the output of the field sync correlators 206. The energy extractor208 can square and sum real and imaginary components received from thefield sync converter 206 to obtain the detected power of the receivedsignal. The detected power of the received signal can be continuallyobtained over time.

In some implementations, the detected power of the received signal canbe combined with a time delayed version of the detected power of thereceived signal. For example, the first 128 bits of a 256 bit datawindow of the detected power can be stored in a delay module 402. Thedelay module can include, for example, a delay register, or array ofdelay registers, that can store the incoming detected power signal for adefined period of clock cycles. In this example, the first 128 bits ofdetected power can be combined with the second 128 bits of detectedpower to form a data window from which a maximum power can bedetermined. The resulting data window is a 256 bit (e.g., 128 delayedbits and 128 real time samples) data window.

In some implementations, the 256 bit data window can be received by amax power detector 404 that can determine the maximum power for the 256bit data window. In turn, this maximum power can be stored in a bin 406that corresponds to the time position of the data window relative to afield time period. For example, the first maximum power that is receivedfrom the max power detector 404 can be stored in a first bin 406 a.Similarly, each subsequent maximum power received from the max powerdetector 404 can be stored in separate bins until the cumulative timerepresented by the data windows satisfies a field time period (e.g., anATSC field time). In some implementations, 4069 bins can be used so thatthe max power for each 256 bit data window in ATSC field can be storedin a separate bin 406.

Once the bins 406 each contain a stored max power that corresponds to a256 bit data window, the cycle repeats, and the next 256 bit data windowcorresponds to the portion of the field time from which the first 256bit data window was sampled. Therefore, the accumulated power of thisportion of the field time can be determined by adding the newly detectedmax power for the current 256 bit data window with the stored max powerthat is contained in bin 406 a. Similarly, each subsequent max power canbe added to a corresponding stored max power so that the power for eachportion of the field time can be accumulated in bins over successivefield times.

Accumulating the max power in the bins 406 over successive field timesreveals the presence of an ATSC field sync signal in a portion of thespectrum 102 being monitored. As discussed, the output of the filed synccorrelators 206 has a greater magnitude when a field sync signal isreceived. Therefore, the detected power of the output of the field synccorrelators 206 will have a greater magnitude when an ATSC field syncsignal is present. This increased power magnitude can be detected by themax power detector 404 and stored in the corresponding bin 406. Overtime, this increased magnitude will accumulate in the corresponding binat a faster rate than the power associated with other portions of thefield time. Thus, identifying the bin 406 that contains the maximumaccumulated power can identify the location of the field sync signal inan ATSC field.

In some implementations, the maximum bin power and power error bars canbe determined by performing a Gumbel distribution based on theaccumulated powers that are stored in the bins 406. A Gumbeldistribution is a particular type of the Fisher-Trippett distributionthat is used to determine a minimum or a maximum of a set of sampleshaving varying distributions. The Gumbel distribution can also be usedto provide an error bar that is associated with the distribution. Oncethe maximum energy is determined, second and fourth moments can beextracted.

§3.0 Example Process Flow

FIG. 5 is a flow chart of an example process 500 of spectrum sensing.The process 500 can be implemented, for example, in the spectrum sensingengine 110 of FIG. 2.

A center frequency of the received digital television signal is shiftedto a reference frequency (502). In some implementations, the receiveddigital television signal is an Advanced Television Systems Committeestandard digital television signal. The center frequency can be shifted,for example, by the modulator 202.

The received digital television signal is filtered to select a portionof the received digital television signal (504). In someimplementations, a 3 MHz portion of the received digital televisionsignal is selected. The received digital television signal can befiltered, for example, by the modulator 202.

A symbol rate of the selected portion of the received digital televisionsignal is converted to a digital television sample rate (506). In someimplementations, the symbol rate can be converted from a system samplerate (e.g., 122.2 MHz) to twice the symbol rate of an ATSC signal (e.g.,2*10.7622 MHz) so that the received digital television signal can beaccurately decoded. The digital television sample rate can be, forexample, a sample rate associated with the Advanced Television SystemsCommittee standard. The sample rate can be converted, for example, bythe sample rate converter 204.

A received digital television signal is correlated to a referencedigital television field sync signal. In some implementations, thecorrelation is non-coherent. The correlation can be performed, forexample, by filtering the received digital television signal with afinite impulse response filter. The finite impulse response filter canhave tap coefficients that correspond to the reference digitaltelevision field sync signal. The correlation can be performed, forexample, by the field sync correlator 206.

A non-coherent correlation power measurement based on the correlation isdetermined (510). In some implementations, the correlation powermeasurement can be determined by determining a windowed maximum power.The non-coherent correlation power can be determined, for example, bythe energy extractor 208.

Maximum non-coherent correlation power measurements over a plurality offield times are accumulated (512). In some implementations, theaccumulation can be performed by binning each of the plurality ofmaximum correlation power measurements into corresponding bins. Theaccumulation can be performed, for example, by the energy extractor 208.

An energy estimate based on the maximum non-coherent correlation powermeasurements is determined (514). In some implementations, the energyestimate can be determined by identifying a maximum accumulated power inthe corresponding bins. In some implementations, the energy estimate canbe determined based on a Gumbel distribution of the non-coherent power.The energy estimate can be determined, for example, by the energyextractor 208.

A transmit mask filter based on the energy estimate is generated (516).In some implementations, the transmit mask filter can be implemented asa finite impulse response filter. The transmit mask filter can beimplemented, for example, by the spectrum sensing engine 110. Thetransmit mask filter is used by a transmitting device so thattransmissions over the portions of the occupied spectrum are precluded.

FIG. 6 is a flow chart of an example process 600 of determining awindowed maximum power. The process 600 can be implemented, for example,in the spectrum sensing engine 110 or the energy extraction engine 208of FIG. 2.

The windowed maximum power of the correlation of the received digitaltelevision signal to the reference digital television field sync signalon a first window of data is determined (602). In some implementations,the windowed maximum power can be determined based on 256 samples ofdata. The windowed maximum power can be determined, for example, by theenergy extraction engine 208.

The windowed maximum power of the correlation of the received digitaltelevision signal to the reference digital television field sync signalon a second window of data is determined (604). In some implementations,the second window of data can include a portion of the data from thefirst window of data. For example, the second window of data can include128 data samples from the first window and 128 new data samples. Themaximum power can be determined, for example, by the energy extractionengine 208.

Embodiments of the subject matter and the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, or in computer software, firmware, or hardware, including thestructures disclosed in this specification and their structuralequivalents, or in combinations of one or more of them. Embodiments ofthe subject matter described in this specification can be implemented asone or more computer program products, i.e., one or more modules ofcomputer program instructions encoded on a tangible program carrier forexecution by, or to control the operation of, data processing apparatus.The computer-readable medium can be a machine-readable storage device, amachine-readable storage substrate, a memory device, a composition ofmatter effecting a machine-readable propagated signal, or a combinationof one or more of them.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, or declarative orprocedural languages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A computer program does notnecessarily correspond to a file in a file system. A program can bestored in a portion of a file that holds other programs or data (e.g.,one or more scripts stored in a markup language document), in a singlefile dedicated to the program in question, or in multiple coordinatedfiles (e.g., files that store one or more modules, sub-programs, orportions of code). A computer program can be deployed to be executed onone computer or on multiple computers that are located at one site ordistributed across multiple sites and interconnected by a communicationnetwork.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Moreover, a computer can be embeddedin another device, e.g., a mobile telephone, a personal digitalassistant (“PDA”), a mobile audio or video player, a game console, or aGlobal Positioning System (“GPS”) receiver, to name just a few.

Computer-readable media suitable for storing computer programinstructions and data include all forms of non-volatile memory, mediaand memory devices, including by way of example semiconductor memorydevices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks,e.g., internal hard disks or removable disks; magneto-optical disks; andCD-ROM and DVD-ROM disks. The processor and the memory can besupplemented by, or incorporated in, special purpose logic circuitry.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinvention or of what may be claimed, but rather as descriptions offeatures that may be specific to particular embodiments of particularinventions. Certain features that are described in this specification inthe context of separate embodiments can also be implemented incombination in a single embodiment. Conversely, various features thatare described in the context of a single embodiment can also beimplemented in multiple embodiments separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Particular embodiments of the subject matter described in thisspecification have been described. Other embodiments are within thescope of the following claims. For example, the actions recited in theclaims can be performed in a different order and still achieve desirableresults. As one example, the processes depicted in the accompanyingfigures do not necessarily require the particular order shown, orsequential order, to achieve desirable results. In certainimplementations, multitasking and parallel processing may beadvantageous.

What is claimed is:
 1. A method of identifying digital televisiontransmissions, comprising: for each of a plurality of digital televisionfield sync time periods: determining, for a first window of datarepresenting a signal received over a first portion of the field synctime period, a first non-coherent correlation power measure based on acorrelation of the first window of data to a reference digitaltelevision field sync signal; storing data representing the firstnon-coherent power measure in a delay register to create a delayed datasample; determining, for a second window of data representing a signalreceived over a second portion of the field sync time period, a secondnon-coherent correlation power measure based on a correlation of thesecond window of data to the reference digital television field syncsignal; and combining data representing the second non-coherentcorrelation power measure with the delayed data sample to createcombined power data; and determining, based on the combined power data,that one of the digital television field sync periods includes a fieldsync signal.
 2. The method of claim 1, comprising generating a transmitmask filter to attenuate transmissions in a portion of spectrumcorresponding to a digital television channel associated with thedigital television field sync period that includes the field syncsignal.
 3. The method of claim 1, further comprising: shifting a centerfrequency of the signal received to a reference frequency for a digitaltelevision channel; filtering the signal received over the digitaltelevision channel to select a portion of the signal; and converting asample rate of the portion of the signal received over the digitaltelevision channel to a system sample rate.
 4. The method of claim 1,wherein determining that one of the digital television field syncperiods includes the field sync signal comprises: for each of one ormore of the digital television field sync time periods: accumulating thecombined power data over multiple periods corresponding to the digitaltelevision field sync time period; determining an energy estimate basedon the accumulation of the combined power data; and determining, basedon the energy estimates for the one or more digital television fieldsync time periods, that one of the one or more digital television fieldsync time periods includes the field sync signal.
 5. The method of claim4, wherein accumulating the combined power data over the plurality offield times comprises binning the combined power data for the pluralityof field time periods into corresponding bins.
 6. The method of claim 5,wherein determining an energy estimate based on the accumulation of thecombined power data comprises identifying a maximum accumulated power inthe corresponding bins.
 7. The method of claim 6, wherein determiningthat one of the digital television field sync periods includes a fieldsync signal comprises determining that the maximum accumulated powermeets a threshold power.
 8. A system, comprising: a field synccorrelator configured to: receive a signal over a plurality of digitaltelevision field sync time periods; and for each of the plurality ofdigital television field sync time periods: determine, for a firstwindow of data representing a signal received over a first portion ofthe field sync time period, a first non-coherent correlation powermeasure based on a correlation of the first window of data to areference digital television field sync signal; and determine, for asecond window of data representing a signal received over a secondportion of the field sync time period, a second non-coherent correlationpower measure based on a correlation of the second window of data to thereference digital television field sync signal; an energy extractorcoupled to the field sync correlator configured to: store datarepresenting the first non-coherent power measure in a delay register tocreate a delayed data sample; combine data representing the secondnon-coherent correlation power measure with the delayed data sample tocreate combined power data; and determine, based on the combined powerdata, that one of the digital television field sync periods includes afield sync signal.
 9. The system of claim 8, further comprising atransmit mask filter to attenuate transmissions in a portion of spectrumcorresponding to a digital television channel associated with thedigital television field sync period that includes the field syncsignal.
 10. The system of claim 8, further comprising: a modulator toshift a center frequency of the signal received to a reference frequencyfor a digital television channel and filter the received digitaltelevision signal to select a portion of the signal; and a sample rateconverter coupled to the modulator to convert a sample rate of theportion of the signal received over the digital television channel to asystem sample rate.
 11. The system of claim 8, wherein the energyextractor is configured to determine that that one of the digitaltelevision field sync periods includes a field sync signal by beingconfigured to perform operations including: for each of one or more ofthe digital television field sync time periods: accumulating thecombined power data over multiple periods corresponding to the digitaltelevision field sync time period; determining an energy estimate basedon the accumulation of the combined power data; and determining, basedon the energy estimates for the one or more digital television fieldsync time periods, that one of the one or more digital television fieldsync time periods includes the field sync signal.
 12. The system ofclaim 11, wherein the energy extractor is configured to accumulate thecombined power data over the plurality of field times by beingconfigured to perform operations including binning the combined powerdata for the plurality of field time periods into corresponding bins.13. The system of claim 12, wherein the energy extractor is configuredto determine an energy estimate based on the accumulation of thecombined power data by being configured to perform operations includingidentifying a maximum accumulated power in the corresponding bins. 14.The system of claim 13, wherein the energy extractor is configured todetermine that one of the digital television field sync periods includesa field sync signal by being configured to perform operations includingdetermining that the maximum accumulated power meets a threshold power.15. The system of claim 14, wherein the energy extractor is configuredto identify the maximum accumulated power by being configured to performoperations comprising performing a Gumbel distribution analysis on theaccumulated power in the corresponding bins.
 16. The system of claim 8,wherein the energy extraction engine is further operable to extract thesecond and fourth moments from the energy estimate.
 17. The system ofclaim 8, further comprising a transmit mask filter to control an outputof a transmitter based on the determination that one of the digitaltelevision field sync periods includes a field sync signal.
 18. Adevice, comprising: a field sync correlator configured to: receive asignal over a plurality of digital television field sync time periods;and for each of the plurality of digital television field sync timeperiods: determine, for a first window of data representing a signalreceived over a first portion of the field sync time period, a firstnon-coherent correlation power measure based on a correlation of thefirst window of data to a reference digital television field syncsignal; and determine, for a second window of data representing a signalreceived over a second portion of the field sync time period, a secondnon-coherent correlation power measure based on a correlation of thesecond window of data to the reference digital television field syncsignal; means for storing data representing the first non-coherent powermeasure in a delay register to create a delayed data sample; means forcombining data representing the second non-coherent correlation powermeasure with the delayed data sample to create combined power data; andmeans for determining, based on the combined power data, that one of thedigital television field sync periods includes a field sync signal. 19.The device of claim 18, further comprising a transmit mask filter tocontrol an output of a transmitter based on the determination that oneof the digital television field sync periods includes a field syncsignal.